1. Field of the Invention
This invention relates to a coated cemented carbide alloy which is very excellent in toughness as well as wear resistance and which is used for cutting tools and wear resistance tools.
2. Description of the Prior Art
A surface-coated cemented carbide comprising a cemented carbide substrate and a thin film such as titanium carbide, coated thereon by vapor-deposition from the gaseous phase, has widely been used for cutting tools and wear resistance tools with higher efficiency. This is superior to the non-coated cemented carbides of the prior art, because the substrate exhibits the high toughness and the surface has excellent wear resistance.
Of late, an increase in the efficiency of the cutting efficiency has been achieved. Since the cutting efficiency is determined by the product of the cutting speed (V) and the feed quantity (f) and since an increase of V causes elevation of the edge temperature, resulting in rapid shortening of the tool life, it has hitherto been proposed to increase the feed f to improve the cutting efficiency. In this case, however, a substrate having a higher toughness is required for dealing with the high cutting stress. To this end, there have been developed alloys wherein the quantity of a binder phase (Co) is increased and wherein the quantity of Co is increased only in the surface layer of the alloy. Moreover, a increase of the cutting speed (V) has lately been taken into consideration in respect to the feed f. In this case, there arises a problem that when the quantity of Co is increased, deformation of the cutting edge is increased at a higher cutting speed, so the tool life is shortened, while when that of Co is decreased, breakage tends to occur at a higher feed quantity (f).
As a wear resistance and impact resistance tool, WC-Co alloys have been used and improvement of the wear resistance or toughness thereof has been carried out by controlling the grain size of WC powder and the quantity of Co, in combination. However, the wear resistance and toughness are conflicting properties, so if Co is increased so as to produce a high toughness in the above described WC-Co alloy, the wear resistance is lowered.
Therefore, the use of WC-Co alloys as a wear and impact resisting tool is necessarily more limited as compared with HSS type alloys (abbreviation of high speed steel). Thus, alloys obtained by replacement of Co thereof by Ni or replacement of WC thereof by (Mo, W)C have also been taken into consideration but the fundamental problems have not been solved.
Japanese Patent Laid-Open Publication No. 179846/1986 discloses an alloy in which the .eta. phase is allowed to be present in the interior of the alloy and a binder phase is enriched outside it. However, this alloy has disadvantages in that because of the presence of a brittle phase, i.e., .eta. phase inside, the impact resistance, at which the present invention aims, is lacking and when the quantity of the binder phase is high in this alloy, dimensional deformation tends to occur due to reaction with a packing agent such as alumina.